Abstract
Estuaries in Southwest Florida experience highly altered freshwater inflow resulting in part from anthropogenic activities. To gage possible effects of altered salinity regimes on species in these systems, physiological responses to salinity stress were investigated in the mud crab Eurypanopeus depressus, a dominant member of oyster-reef communities in the region. Haemolymph osmolality and oxygen consumption were measured on animals acclimated to 5‰, 15‰, 25‰, and 35‰ over a period of four weeks in a laboratory setting. Haemolymph osmolality varied significantly with respect to salinity, with measured osmolalities at each salinity treatment being significantly different (p<0.0001) from one another. Oxygen consumption also varied significantly with salinity, with values for the lowest salinity treatment (5‰) being generally higher (p<0.0001) than all other treatments over the length of the study. This general pattern of increased oxygen consumption in response to dilute salinity levels is a trend that has been documented preciously in several estuarine species. The results of this study indicate that osmoregulation requires greater energy expenditure by Eurypanopeus depressus at subnormal salinity levels. Having to expend additional energy to meet osmoregulatory demands can result in reduced amounts of energy for processes such as growth and reproduction, thereby affecting organism health. Suboptimal salinity levels resulting from altered freshwater inflow can thus significantly affect species abundance and distribution, especially in the upper reaches of estuaries.